1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) signal communication system used in broadband mobile communication and the like, which divides transmission signals into subcarrier groups orthogonal to each other to perform multi-carrier transmission. More specifically, the invention relates to an OFDM signal communication system which achieves substantial frequency utilization efficiency under a multipath fading environment, using a plurality of transmitting antennas and a plurality of receiving antennas, and which uses a space division multiplexing (SDM) method or polarization division multiplexing (PDM) that can achieve signal transmission with high quality, high capacity, and high speed.
2. Description of the Related Art
For example in broadband mobile communication, since the frequency band which can be used is limited, then in order to deal with full-scale multi-media, it is necessary to achieve high frequency utilization efficiency on a par with fixed microwave communication, and overcome severe frequency selective fading, to realize high quality transmission.
In order to realize high capacity/high speed mobile communication, using a limited frequency band, the following method is proposed. That is to say, a multiple input multiple output (MIMO) channel is set up using a plurality of transmitting antennas and a plurality of receiving antennas, and on the transmitter, a plurality of channels are transmitted using the same frequency, while on the receiver, each of the channels is separated by an equalizer and an interference canceller to achieve a large capacity.
In an MIMO Rayleigh fading channel which is formed in the case of using N transmitting antennas on the transmitter and M (M≧N) receiving antennas on the receiver, the Shannon's capacity limit is expressed by the following equation.
                    C        =                              log            2                    ⁡                      [                          det              ⁡                              (                                  I                  +                                                            ρ                      N                                        ⁢                                          H                      ·                                              H                        *                                                                                            )                                      ]                                              (        1        )            
Here H is an M×N matrix and the elements (i,j) thereof are the propagation coefficients between the i th transmitting antenna and the j th receiving antenna. Furthermore, I is the M×N eigen-value matrix and ρ is the mean SNR. Furthermore, det is the determinant, and * denotes the complex conjugate. When M=N, the lower limit of the capacity is expressed by the following equation.
                    C        =                              ∑                          k              =              1                        N                    ⁢                                    log              2                        ⁡                          (                              1                +                                                      ρ                    N                                    ⁢                                      χ                                          2                      ⁢                      k                                        2                                                              )                                                          (        2        )            
Here χ22k shows the effect of the diversity with an order of k. That is to say, for an MIMO channel, the capacity is N times that of a single channel. In this manner, in an MIMO channel, if an ideal interference cancellation is achieved, then in the broadband mobile communication, large capacity and high speed transmission can be realized.
A configuration example of a conventional transmitter-receiver for this MIMO channel is shown in FIG. 37. This is a configuration example of a transmitter-receiver which performs time-space equalization using N transmitting antennas 1110-1 to 1110-N, and N receiving antennas 1111-1 to 1111-N. On the transmitter, the transmission information is coded in encoders 1101-1 to 1101-N, interleaved by interleavers 1102-1 to 1102-N, and distributed to N modulators 1103-1 to 1103-N, and then transmitted.
On the other hand, on the receiver, N−1 interference cancellers 1114-1 to 1114-(N−1), and N equalizers 1115-1 to 1115-N are arranged. The received signal of the receiving antenna 1111-1, is at first equalized by the equalizer 1115-1, and then deinterleaved by a deinterleaver 1116-1, and input to a decoder 1118-1. In the decoder 1118-1 decoding is performed corresponding to encoding by the encoder 1101-1.
The interference component is extracted by calculating the difference of the output from the decoder 1118-1 and the output from the deinterleaver 1116-1. This interference component is input to the interleaver 1117-1, and the output therefrom is fed back as a control signal to the equalizer 1115-1. On the other hand, the interference component, being the output from the interleaver 1117-1 is subtracted from the output from the equalizer 1115-1, and again input to the deinterleaver 1116-1.
By means of this repetitive processing, the reliability of the output from the decoder 1118-1 is increased. At the receiving antenna 1111-1, the N transmission signals from the transmitting antennas 1110-1 to 1110-N are all superposed and received. In the interference canceller 1114-1, the output from the decoder 1118-1 is subtracted from the received signal of the receiving antenna 1111-1 for which all the N transmission signals have been superposed.
As a result, the signal transmitted by the transmitting antenna 1110-1 is removed from the signal received by the receiving antenna 1111-1, to give a signal in which the (N−1) transmission signals of the transmitting antennas 1110-2 to 1110-N are superposed. This signal is input to the next equalizer 1115-2. In the equalizer 1115-2, as with the processing by the system of the equalizer 1115-1, after being equalized by the equalizer 1115-2, the signals are deinterleaved by the deinterleaver 1116-2, and input to the decoder 1118-2.
In the decoder 1118-2, decoding corresponding to the encoding by the encoder 1101-2 is performed. The interference component is extracted by calculating the difference of the output from the decoder 1118-2 and the output from the deinterleaver 1116-2. This interference component is input to the interleaver 1117-2, and the output therefrom is fed back as a control signal to the equalizer 1115-2. On the other hand, the interference component, being the output from the interleaver 1117-2 is subtracted from the output from the equalizer 1115-2, and again input to the deinterleaver 1116-2.
By means of this repetitive processing, the reliability of the output from the decoder 1118-2 is increased. In the interference canceller 1114-2, the output from the decoder 1118-2 is subtracted from the input from the decoder 1118-1. As a result, the signal transmitted by the transmitting antenna 1110-2 is further removed to give a signal in which the (N−2) transmission signals of the transmitting antennas 1110-3 to 1110-N are superposed.
This signal is input to the next equalizer 1115-3 (not shown in the figure). In this manner, the interfering signals decoded by the decoder 1118 are removed sequentially by the interference canceller 1114, and the output from the interference canceller 1114-(N−1) finally becomes the transmission signal of the transmitting antenna 1110-N and is equalized by the equalizer 1115-N, deinterleaved by the deinterleaver 1116-N and decoded by the decoder 1118-N. This operation is performed for the receiving antennas 1111-2, 1111-3 (not shown in the figure), and 1111-N.
The decoding result from the respective decoders 1118-1 to 1118-N, is repetitively processed in series, and finally the outputs from the N decoders are sent to a converter 1119, and converted to serial received data. This is equivalent to estimating a propagation coefficient matrix for the respective paths between the transmitting antenna 1110-i and the receiving antenna 1111-j by the equalizer, and performing interference cancellation based on this.
Consequently, for the operation of the equalizer, it is necessary to equalize the N×N paths, and perform (N−1)×N interference cancellations based on the result.
For the transmitter-receiver in the conventional MIMO channel of FIG. 37, N equalizers are necessary for each of the respective receiving systems corresponding to the receiving antennas of the receiver. Furthermore, in the case where broadband transmission is performed in a severe multipath fading environment, frequency selective fading occurs, so that it is necessary to accurately identify the frequency characteristics for the amplitude and phase which are generated by the fading, in an extremely short time for each of the systems.
However, in the current fading environment, the number and strength of the incoming delayed waves, and the so called delay profiles are diverse, and for all of these environments, realization of an effective equalizer is extremely difficult. Therefore, although the transmitter-receiver in the MIMO channel can be realized under an environment close to the Additive White Gaussian Noise channel such as for point-to-point communication, in a MIMO channel in severe multipath fading environments, an extremely large signal processing capability is necessary. Therefore realization of a conventional transmitter-receiver in the MIMO channel is difficult.
Furthermore, in the conventional transmitter-receiver in the MIMO channel as in FIG. 37, interference cancellation is performed by estimating the frequency characteristics of the amplitude and phase which are distorted by the multi-path fading, regenerating the interference replica, and subtracting the replica from the output from the decoder 1118-1. In this case, in the respective equalizers, high estimation accuracy is necessary for the frequency characteristics of the amplitude and phase. This is because, in the case where equalizing accuracy cannot be achieved, the interference cancellation is not sufficient, resulting in residual interference noise.
However, with the equalizer, since high accuracy equalization for the frequency characteristics of the amplitude and phase is difficult, there is a problem in that the signal-to-interference noise ratio easily deteriorates.
Furthermore, in the case where a wireless communication system is assumed between a fixed based station and a mobile terminal, if complicated processing functions are provided in the mobile terminal side, a problem arises from the point of miniaturization and low cost for the mobile terminal due to increase in hardware size and increase in power consumption.